Performance of a Rotating Magnetic Field Thruster

Abstract

The rotating magnetic field (RMF) thruster is a type of inductive pulsed propulsion device theoretically suited for high power operation. It utilizes a rotating magnetic field to produce an azimuthal current in the presence of a steady background field with a radial gradient to eject plasma at high velocity and repetition rates. Historically, performance of RMF thrusters has been low. The highest observed efficiency was 8%. To better understand the mechanisms driving this low performance, the University of Michigan’s Plasmadynamics and Electric Propulsion Laboratory (PEPL) developed an RMF test article capable of operating over a number of conditions. The thruster was tested at power levels at or below 4 kW. The thrust produced specific impulses up to 500 s at efficiencies less than 2%. A lumped circuit model approach was then used to predict performance of the thruster in order to gain greater insight into the thrust and loss mechanisms. The equivalent circuit was derived by modeling the driving antennae and plasma as a collection of current loops interacting via mutual inductance and Lorentz forces. Several physically relevant assumptions were applied to reduce the complexity of the system. The resulting set of equations require six free circuit parameters that must be determined experimentally. Data from the experiment was used to calibrate the model. Broadly, the model predictions agree well with measured performance for specific impulse and efficiency. Extrapolating the model results beyond experimental data reveals key scaling characteristics of the thruster. At lower specific energies, the efficiency initially increases rapidly. However, at 500 - 1,000 J/mg (depending on plasma resistance), it reaches a maximum before decreasing gradually with increasing specific energy. Increasing the specific energy yields higher exhaust velocities, although the rate of increase decreases after the peak efficiency is reached. Coupling efficiency drops rapidly with increasing specific energy. Additionally, a minimum of 99% of all energy coupled to the plasma is lost. The results of the model illuminate several pathways to increase performance including limited increases in specific energy, increasing background magnetic field strength, increasing the thruster characteristic size to ~ 1 m, and limiting pulse length to match the plasma residence time.